When you have multiple versions of Exchange Server installed during a migration, you might need to specify which version of ECP you want to use. For example, migrating for Exchange 2013 to Exchange 2019 you might want to use one or the other. By default, when you sign in, you are directed to the version where your mailbox resides.
For users without mailboxes, they are redirected to the version of Exchange where the arbitration mailboxes are located. So, it's generally best to move the arbitration mailboxes to the newest version of Exchange server early in your migration process.
When you access ECP, you can specify which version of ECP you want to use as part of the URL by adding ?ExchClientVerXX. For example, https://mail.contoso.com/ecp?ExchClientVer=15.
You can use the following version numbers:
- 14 - Exchange 2010
- 15 - Exchange 2013
- 15.1 - Exchange 2016
- 15.2 - Exchange 2019
References:
